1. Field of the Invention
The present invention relates to a method and to an apparatus for the detection and/or qualitative and/or quantitative imaging of measuring points which carry a voltage of at least a specific frequency with the aid of an electron microscope in which the measuring point is subjected to primary electrons, in which secondary electrons are released by the primary electrons at the measuring point and are detected in a detector and in which a secondary electron signal resulting therefrom is further processed by way of a lock-in amplifier into a measuring signal.
2. Description of the Prior Art
In the case of faulty behavior of integrated circuits, there is a desire, for the purpose of locating a defect, to compare the behavior of the integrated circuit with the nominal behavior, such as results from simulation. To this end, it must be checked inter alia which frequency or which frequency spectrum a signal exhibits at a measuring point within the integrated circuit.
From the prior art up to the present, three methods are known with which it is possible to check whether specific internal periodic signals of a specific frequency are present at a measuring point within an integrated circuit. From the publication "Scanning Electron Microscopy", 1975, Part I, Proc. of the Eighth Annular Scanning Electron Microscope Symposium, Chicago, IIT Research Institute, pp. 465-471, the method of the so-called "voltage coding" is known. The voltage coding method images the dynamic voltage distribution of the integrated circuit on a television monitor. The voltage coding method makes possible a chronological allocation of the switching states in the various components, and is therefore particularly suited for the rapid operational check of integrated circuits. However, the voltage coding method has grave disadvantages which inevitably results from the line frequency of the electron beam.
From the U.S. Pat. No. 4,223,220, fully incorporated herein by this reference, a so-called "logic state mapping" method is known. In the logic state mapping method, the dynamic voltage distribution is imaged with the aid of a stroboscope effect. The logic state mapping method compared with the voltage coding method, given the same voltage resolution, delivers a time resolution greater by orders of magnitude. The logic state mapping method, moreover, simplifies the recording, since the images of the dynamic voltage distribution can be directly photographed from the photo display screen of a scanning electron microscope. In the case of the voltage coding method, by contrast, a recording of the dynamic voltage distribution is possible only with a tape memory or with photos from a television monitor.
According to a third method disclosed by J. P. Collin in the publication Proceedings of Journee d'Electronique, 1983, "Testing Complex Integrated Circuits: A Challenge", published by the Swiss Federal Institute of Technology, Lausanne, Switzerland, pp. 283-298, entitled "Un Alternative Economique au Contraste de Potentiel Stroboscopique: LeTraitement du Signal d'Electrons Secondaires d'un Microscope a Balayage", the location of specific frequencies at a measuring point within an integrated circuit is carried out pursuant to utilization of a lock-in amplifier. From a voltage contrast signal obtained at a measuring point within the integrated circuit, with the aid of a lock-in amplifier, a signal with the sought frequency is filtered out and then the intensity of the signal is imaged as a variation in brightness.
The three methods set forth above, such as are known from the prior art, can be carried out only with difficulty in part and, in part, permit only a check of some few interconnections in the integrated circuit. Moreover, the methods known from the prior art assume that one knows the frequency of a signal to be sought, or that a signal synchronous with the signal to be sought is accessible from the exterior. If the frequency of a sought-for signal is unknown, the search for such a signal becomes very laborious and expensive. This is the case, for example, when the frequency of the signal to be sought is obtained through a division by 255 instead of through a division by 256.